What is the reason a small loop-stick antenna ( typically used for AM reception) does not have reciprocal characteristics for both reception and transmission. ( AM reception works great on those little loopsticks, but you don't get desirable results, when trying to transmit back through such a device)


2 Answers 2


At reasonable transmission powers, the ferrite core saturates. Also, it would probably overheat, and produce a horribly distorted transmission.

The ferrite core is made of movable magnetic domains. Each of these are small bits of the material that have magnetic poles like a tiny bar magnet. Normally they are all pointing in random directions, and their individual fields cancel so the ferrite bar doesn't seem to be a magnet. A lot of matter is like this.

Ferrite is special in that the magnetic domains are easily moved. When an external magnetic field is applied, they all move to align with that external field. Then their fields add to the external field, and you end up with a magnetic field which is stronger than the external field would have been if the ferrite were replaced with air.

This property is called magnetic permeability. It's this magnetic "amplification" that makes the loopstick antenna so effective. The ferrite stick increases the antenna aperture by concentrating the magnetic flux from far away transmitter through the center of the windings where it will induce a current that is detected by the receiver.

The trouble is the magnetic domains can become only so much aligned. At some field strength they are as aligned as they can be, and the permeability drops to zero.

The ferrite doesn't care if the external magnetic field is coming from a distant transmitter, or the coil around it. Because the field from the transmitting coil is many orders of magnitude stronger, saturation of the core is likely unless the transmit power is very small, maybe microwatts.

This saturation of the core means the antenna is highly nonlinear, and would thus introduce unacceptable distortion into the transmission.

Furthermore, the magnetic domains are "sticky": it takes some amount of energy that's wasted as heat to move them. This is called hysteresis loss. When receiving this loss reduces the efficiency of the antenna, but since the field is so weak there's no significant heating. When transmitting the energy lost is higher, and without adequate cooling heat may accumulate to reach the ferrite's Curie temperature, and the ferrite's magnetic properties will be lost.


The MW AM band is full of noise - man made, thermal etc. The loopstick is not efficient so it picks up a small amount of signal and a small amount of noise. The incoming signal-to-noise ratio (SNR) of the MW AM antenna will have little to do with antenna type. The incoming noise at these frequencies is greater than the noise produced by electronics in the receiver. So the MW AM receiver will perform fine on any short antenna because the overall RF/IF gain is more than enough. These sweeping statements about noise are definitely not true at 2GHZ. So the MW receiver works fine on a loopstick or car radio whip or any electrically short antenna. For transmitting, the losses in the loopstick would have to be made up in increased transmit power. This would be expensive. The ferrite would have to be big to avoid saturation that Phil Frost warned about. Using a loopstick for TX is like reducing transmit power, which will reduce range. If there was no noise of any kind then RX gain like say a preamp would be just as effective at increasing range as say a TX power increase like say a linear. So this is why you see big towers for AMBC and not big loopsticks.


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